The rotation velocity is usually expressed as the length of day (lod); it contains periods of 1 year and shorter, but also (though they are weaker) longer periods from 11 yr to 18.6 yr [1]}, [2]}, [3]}, [4]}, [5]}. [6]} showed that the secular variation of lod since 1846 [7]} and RP have been carried by an oscillation whose period fits the Gleissberg cycle.
